This invention relates to the production and treatment of geothermal brine, and more particularly, to the separation and recovery of heavy and transition metals present in produced geothermal brines.
As conventional hydrocarbon fuel reserves have been depleted, a worldwide emphasis has been placed on developing alternative energy resources, such as geothermal energy. Geothermal energy is generated by producing steam, hot water or hot aqueous brines from a subterranean geothermal reservoir and utilizing the produced fluid in conjunction with surface equipment, such as turbines and heat exchangers, to derive useful energy therefrom.
Most hot aqueous geothermal brines which are produced from subterranean formations for the purpose of energy derivation contain substantial quantities of salts such as sodium chloride, calcium chloride and potassium chloride. These hot aqueous brines may also contain substantial quantities of dissolved heavy and transition metal ions, such as silver, copper, lead, zinc, manganese and iron, and are usually saturated with silica. As these brines are produced via wells penetrating a subterranean geothermal reservoir, the pressure thereon is reduced and the brines begin to boil or flash thereby forming a two phase mixture of liquid and vapor. Commensurate therewith is a temperature drop which causes the precipitation and deposition of heavy and transition metal sulfides and some precipitation of silica resulting in fouling of the processing piping and equipment. Further, since most of the sulfide ion, present as hydrogen sulfide, is immediately separated from the brine along with other noncondensible gases in a separator at the wellhead, approximately less than 1 weight percent of the metal ions present in the produced geothermal brine are precipitated during production of the brine as just described. Therefore, since the majority of these metal ions remain in solution with the brine they will not only cause subsequent scaling and corrosion problems within the processing apparatus but will also contaminate the brine rendering the recovery and purification of salts therefrom a difficult task. Metals such as iron, manganese and zinc, which usually are the most abundant metals found in the brine, promote deposition of silica scale and may also cause corrosion of the processing apparatus.
In view of this problem, several techniques have been advanced to substantially eliminate scaling resulting from the production of geothermal brines. One process involves adding an agent such as sodium sulfate to a produced, hot geothermal water prior to passing the water through a heat exchanger so as to generate non-scaling precipitates. Such precipitates are readily separated from the hot geothermal water after it has passed through the heat exchanger. It has been proposed to utilize downhole pumps in conjunction with such process to compensate for the pressure drop concomitant with production of geothermal brine and, therefore, substantially eliminate boiling or flashing of the brine.
Another prior art process discloses the selective precipitation and subsequent recovery of certain valuable components of geothermal brines. More particularly, iron, manganese, zinc and lead can be precipitated as hydrous oxides, lithium can be precipitated as a complex lithium aluminate compound, and barium and strontium can be precipitated as sulfate salts. In addition to recovering these valuable components, salts such as sodium chloride and potassium chloride, can be recovered from the brine by suitable means such as, solar evaporation ponds. However, a need exists for an improved process for separating and recovering metal values from geothermal brine which is more efficient and more economical than the prior art processes.
Accordingly, it is an object of the present invention to provide a process for producing and processing hot geothermal brines containing, inter alia, salts and heavy and transition metal ions, wherein uncontrolled precipitation and scaling is substantially reduced.
It is also an object of the present invention to provide a process for the removal and recovery of valuable metals present in hot geothermal brines prior to energy derivation from the brine.
It is a further object of the present invention to provide for the precipitation of heavy and transition metal ions present in produced geothermal brines upon the addition of a single precipitating agent to the brine.
These and other objects and advantages of the invention will be apparent from the following detailed description and drawing.